Within this framework, patent application EP 1 237 369 aims to combat the copying of images by picture-taking during their display, for example with a camcorder in a cinema auditorium. With this aim, it is proposed that the intensity of the pixels of a pattern be modulated about the value to be displayed at a high frequency which renders the pattern invisible to the human eye but which generates artefacts on the sequence filmed by the camcorder. This pattern is commonly called watermarking or anti-copy pattern. The shape of the pattern is determined so as to inscribe for example messages of the type “ILLEGAL COPY” which will appear in the images displayed by the camcorder.
In order for the pattern to be invisible to the naked eye, the modulation consists in alternating images in which the pattern is bright with images in which it is dark, the mean intensity of the pattern over several images corresponding to that to be displayed in the images in the absence of a pattern. During the display of these images, the eye carries out an integration and in fact perceives the mean intensity. This technique may also be applied to the colour of the images by alternating images in which the pattern is more coloured with images in which it is less so, the mean colour of the pattern over several images corresponding to that to be displaced in the absence of a pattern.
In practice, the patterns are contained in a modulation image and the images of the sequence to be protected are modulated with this modulation image.
One of the principal difficulties of this type of technique is to determine the locations at which to place the pattern or patterns in the images so as to maximize the hindrance to a person watching an illicit copy generated for example by a camcorder.